Diffusion transfer photographic processes are well known in the art. Such processes have in common the feature that the final image is a function of the formation of an imagewise distribution of an image-providing material and the transfer by diffusion of image-providing material imagewise to an image-receiving layer. In general, a diffusion transfer image is obtained first by exposing to actinic radiation a photosensitive element, or negative film component, which comprises at least one light-sensitive silver halide layer, to form a developable image. Thereafter, this image is developed by applying an aqueous alkaline processing composition to form an imagewise distribution of diffusible image-forming material, and transferring this imagewise distribution by diffusion to the image-receiving layer of a superposed image-receiving element, or positive film component, to form a transfer image thereon.
The aqueous alkaline processing compositions employed in diffusion transfer processes permeate the emulsion layer(s) of the photosensitive element to effect development thereof, and generally comprise aqueous alkaline compositions having a pH in excess of about 10, and frequently in the order of about pH 12 to about pH 14. The processing compositions utilized in diffusion transfer processes generally comprise at least an aqueous dispersion of a highly alkaline material, for example, potassium hydroxide, sodium hydroxide or the like. The processing composition typically includes a light-reflecting pigment such as, for example, titanium dioxide, a thickening agent which is generally a high molecular weight polymer, e.g., a polyacrylic acid, and optical filter agents, as well as, development accelerators, silver solvents, antifoggants, post-process transfer inhibitors, and inorganic materials, such as, for example, silica.
After processing has been allowed to proceed for a predetermined period of time, it is desirable to neutralize the alkali of the processing composition to prevent further development and image material transfer, and, in some instances, subsequent oxidation which may have a material and substantial effect upon the stability to light of the resulting image in the image-receiving layer. Accordingly, a neutralizing layer, typically a nondiffusible acid-reacting material, is typically employed in the film unit to lower the pH from a first (high) pH of the processing composition to a predetermined second (lower) pH. To ensure that the pH reduction occurs after a sufficient, predetermined period and not prematurely so as to interfere with the development process, a timing layer is typically positioned adjacent the neutralization layer.
Diffusion transfer photographic materials known in the art include those wherein the photosensitive silver halide emulsion layer(s) and the image-receiving layer are initially contained in separate elements which are brought into superposition prior or subsequent to, exposure. Alternatively, the photosensitive layer(s) and the image-receiving layer may initially be in a single element wherein the photosensitive and image-receiving components are retained together in an integral negative-positive structure. In either case, after development the two elements may be retained together in a single film unit, commonly referred to as an integral film unit, or separated, commonly referred to as a peel-apart film unit.
The transfer image of an integral film unit may be viewed through a transparent support against a reflecting background, such as, for example, that provided by a dispersion of a white, light-reflecting pigment, e.g., titanium dioxide, and the layer providing the reflecting background is generally positioned between the developed silver halide emulsion layer and the image-receiving layer. As would be appreciated by those of skill in the art, several embodiments of integral film units are known, including, for example, those described in U.S. Pat. Nos. 3,415,644; 3,594,165; 3,647,435; and 3,647,437.
The integral film units described in U.S. Pat. No. 3,415,644 include suitable photosensitive layers and image dye-providing materials carried on an opaque support, an image-receiving layer carried on a transparent support and means for distributing a processing composition between the elements of the film unit. Photoexposure is made through the transparent support and the image-receiving layer, and a processing composition which includes a light-reflecting pigment is distributed between the photosensitive and image-receiving elements. After the distribution of the processing composition and before photographic processing is complete, the film unit can be, and preferably is, transported into the light. Accordingly, in such film units, the layer provided by distributing the light-reflecting pigment must be capable of performing specific and critical assigned functions, including providing suitable protection against further exposure of the photoexposed photosensitive element prior to the completion of processing, allowing suitable effective transfer of the image-forming materials from the photoexposed photosensitive layer to the image-receiving layer, and providing a reflecting background of suitable efficiency for viewing the transferred image. In addition, the reflecting layer serves to mask the developed photosensitive layer(s).
Integral film units described in U.S. Pat. No. 3,594,165 include a transparent support carrying suitable photosensitive layers and associated image dye-providing materials, a permeable opaque layer, a permeable light-reflecting pigment-containing layer, an image-receiving layer viewable through a transparent support against the light-reflecting layer, and means for distributing a processing composition between the photosensitive layer and a transparent cover or spreader sheet. Moreover, the processing composition is opaque and is distributed after photoexposure to provide a second opaque layer which can prevent additional exposure of the photosensitive element. In such film units exposure is made through the transparent support sheet. After distribution of the processing composition and formation of the second opaque layer, the film unit typically is transported into the ambient light before processing is complete. Accordingly, in such film units, the light-reflecting pigment-containing layer also performs the critical tasks referred to above for the film units described in U.S. Pat. No. 3,415,644, and masks the developed photosensitive layer.
It is also well known in the art such as, for example, is described in U.S. Pat. Nos. 3,647,435 and 3,647,437, to include auxiliary opacification systems in such integral film units. The auxiliary opacification systems are designed to cooperate with the light-reflecting layer and/or light-reflecting materials referred to above to provide even more opacity to prevent further exposure of the film unit through the light-reflecting layer during the photographic processing of the film unit in the ambient light.
For example, U.S. Pat. No. 3,647,437 describes an auxiliary opacification system that employs a pH-sensitive, optical filter agent which can absorb light at one pH but is rendered less light-absorbing at another pH. More particularly, the optical filter agent is dispersed in the processing composition together with the light-reflecting pigment. The processing composition is integrated with the elements of the film unit so that the processing composition can be distributed between the photoexposed photosensitive layer and the image-receiving layer. Accordingly, after the distribution of the processing composition, an opaque layer comprising the light-reflecting pigment and the optical filter agent is provided and the opaque layer covers a substantially major surface of the photoexposed layer. During the initial stages of development, the pH-sensitive optical filter agent absorbs light and cooperates with the reflecting pigment to provide a degree of opacity sufficient to prevent substantially any additional photoexposure through the layer. As the transfer of the image-forming materials proceeds, the light-absorbing capability of the pH-sensitive optical filter agent is reduced until the optical filter agent becomes substantially non light-absorbing and its opacification function is terminated. When the transfer of the image-forming material is complete, the light-reflecting layer comprising the light-reflecting pigment and the non light-absorbing optical filter agent provides a reflecting background for viewing the final image.
Importantly, as would be understood by those of skill in the art, to realize the critical functions of the light-reflecting layers described above, the light-reflecting materials comprising the light-reflecting layer must remain substantially uniformly dispersed within the layer distributed from the processing composition and within the processing composition itself prior to and during the distribution thereof. Hence, because premature interactions between the various components of the processing composition resulting in, for example, agglomeration of such components, would interfere with the functions of the composition it is desirable to keep such components uniformly suspended.
Since the processing composition is typically provided in a rupturable container which is a component of the photographic film unit, the composition is required to retain its desired rheological properties during the shelf-life of the film unit. Further, efforts to eliminate the agglomeration of the components in aqueous alkaline processing compositions can be difficult without detrimentally affecting the photographic quality of diffusion transfer film units. In other words, due to the complexity of the chemical interactions between the constituents of the processing composition, as well as, the interactions of the processing composition with the other components of the film unit, including the timely provision of suitable opacification, minor changes in the formulation of the processing composition can have a significant impact on performance of the the resulting photographic product.
There are described various techniques for keeping components of the aqueous alkaline processing compositions dispersed during storage and also for maintaining dispersions of pigments in liquid organic media such as pigmented lacquers and plastic materials. Similarly, in the paint industry, it is known to use a dispersion of titanium dioxide to influence the properties of the products in regard to hue, gloss and physical and chemical behavior. To realize these beneficial effects, the titanium dioxide particles must remain substantially dispersed, i.e., uniformly distributed, in the aqueous alkaline solutions or in the liquid organic media to which they are added.
U.S. Pat. No. 3,642,510 describes a process for preparing titanium oxide pigments which are dispersable in hydrophobic systems such as paints or varnishes by adding an alkali metal salt of a high molecular weight carboxyl compound to a titanium oxide slurry finely dispersed in water or an alcohol containing an aluminum salt or zinc salt to form a soap of aluminum or zinc on the surface of the titanium oxide.
U.S. Pat. No. 4,235,768 describes a process of coating a titanium dioxide pigment with an organic polymer containing carboxyl groups to produce a homogeneous dispersion of the pigment in liquid organic media, e.g., pigmented lacquers and plastic materials.
U.S. Pat. No. 4,246,040 describes a method of surface treating a powdery or granular solid substance such as titanium dioxide which comprises reacting a basic polyaluminum salt with an acid or its salt in the presence of the titanium dioxide to alter the hydrophilic or lipophilic properties of the titanium dioxide.
In addition, methods are known in the art to alleviate or circumvent the undesirable results brought about by the settling or agglomeration of titanium dioxide particles in aqueous alkaline processing compositions, e.g., by self-association or interaction with other components of the processing compositions, for example, by encapsulating the titanium dioxide particles such as disclosed in U.S. Pat. No. 3,833,369; or, by adding additional titanium dioxide particles to the processing composition, so that, in effect, the settling out of the titanium dioxide particles still occurs but the amount present overall provides suitable opacification and reflection.
While such materials have been found to provide advantageous results as are described in the above-mentioned patents; nevertheless, their performance in some photographic systems is not completely satisfactory. In some instances, with integral diffusion transfer film units which are ejected from the camera immediately upon distribution of the processing composition and development is allowed to take place in the ambient light, there have been encountered defects in the final image in the form of small localized spots which have been referred to as pinpoint opacification defects. Also, there have been observed defects in the images obtained from other types of diffusion transfer photographic film units which can be attributed to less than uniform spreading of the processing composition. Hence, as the state of the art for photographic systems advances, novel techniques and materials continue to be developed in order to attain the performance criteria required of such materials. There is a need for novel photographic processing compositions, film units and methods for use in diffusion transfer photographic systems that have advantages over those already known to the art; hence, investigations continue to be pursued to provide such advantages.